At our school, 6 years old is the youngest age that we start children in private piano lessons. At this age they have begun to develop longer attention spans and can retain material with ease.
10 years old is recommended as the youngest age for private vocal lessons. Due to the physical nature of voice lessons (proper breathing techniques, development of vocal cords and lung capacity), the younger body is generally not yet ready for the rigors of vocal technique.
Due to lung capacity we recommend that most woodwind beginners are 9 and older.